Arthropoda. 



187 



M 



I, 



indeed, all the ventral ganglia may unite into a single unsegmented 



mass; this is always accompanied by the shortening of the body, 



as in Crabs. Sometimes ganglia are shifted during 



development, so that those belonging to one segment 



move further forward ; but the nerves arising from 



such a pair are distributed to the segment to which 



they properly belong. The members of a pair are 



united by a commissure, which is almost always short, 



often so short that they appear to be fused; this 



is often the case, also, with the connectives of 



consecutive pairs. 



Sense organs. The formation of a cuticular 

 skeleton results in the restriction of the sense of touch 

 to certain spots on the surface of the body. In 

 particular many setae become tactile; beneath the 

 epidermis lie one or more sensory cells, each of 

 which sends a filiform process into the seta from one 

 end, and a nerve fibre from the other end, to the 

 central nervous system. Hairs, provided with a thin 

 cuticle, and occurring upon the first antennae of 

 Crustacea, act as olfactory organs; so also do 

 the peg-shaped processes upon the antennae of Insects 

 {see p. 19) : like the tactile structures, they are con- 

 nected with sensory cells. Auditory organs 

 are found in many Crustacea, and in some Insects ; 

 these will be considered in the several groups. Optic 

 organs, which reach such a high stage of de- 

 velopment among the Arthropoda, appear in two 

 forms; as simple eyes, or ocelli, and as compound 

 eyes. The most important points in the structure 

 of these eyes have already been noticed in the General 

 Part, pp. 21, 22. In most of these animals there is a 

 pair of compound eyes, as well as several ocelli, but 

 in others ocelli only are developed. 



The digestive tract usually runs through 

 the body as a tolerably straight tube ; the mouth is at 

 the anterior end, and is usually ventral; the anus 

 is posterior. Salivary glands and a liver may or may not be present. 



Vascular system. The heart, which is usually tubular, 

 corresponds to the dorsal vessel of the Annelids, and is found on 

 the dorsal side, above the digestive tract. It is furnished with venous 

 ostia, generally several pairs, through which the blood enters the 

 heart from the surrounding blood-space, the pericardium: the 

 pericardium receives the blood from the gills (lungs) when such are 

 present, or from the body. In other respects, the vascular system of 



Fig. 149. Ner- 

 vous system of 



Gammarus. 

 c "brain, o eye, 

 a first abdominal 

 ganglia, I first 

 thoracic ganglia. 

 — After Sars. 



